Distillation of tetramethyl lead-acetonitrile azeotrope with thermal stabilizer



Jan. 9, 1968 J N N 3,362,889

DISTILLATION OF TETRAMETHYL LEAD-ACETONITRILE AZEOTROPE WITH THERMALSTABILIZER Filed Feb. 24, 1966 STABILIZER STORAGE PRODUCT STREAM sonomsSTREAM INVENTOR JOHN F. HANNAN BY We. 4

ATTORNEY United States Patent 3,362,889 DISTILLATION 0F TETRAMETHYLLEAD- ACETONITRILE AZEOTROPE WITH THER- MAL STABILIZER John FrancisHannan, Wilmington, DeL, assignor to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware Filed Feb. 24,1966, Ser. No. 529,879 4 Claims. (Cl. 203-6) ABSTRACT OF THE DISCLOSUREDistillation of tetramethyl lead-acetonitrile azeotropes with theaddition of a thermal stablizer at the top of the distillation column.

Background of the invention This invention relates to the distillationof tetramethyl lead-acetonitrile azeotropes, and more particularly, to anovel method of stabilizing tetramethyl lead during distillation oftetramethyl lead-acetonitn'le azeotropes in which the thermal stabilizeris introduced separately into the distillation unit.

Tetramet-hyl lead is well known as a valuable antiknock agent for motorfuels. It has a normal boiling point of 110 C., but is highly unstableat elevated temperatures. Overheating can lead to violent decomposition.However, methods are known for reducing this thermal decompositionhazard. For example, in US. Patent 3,049,558, Cook et a1. disclosescompounds which stabilized tetramethyl lead when added in small amounts.Listed compounds include saturated and unsaturated aliphatic andaromatic hydrocarbons boiling in the range of 90 to 150 C.

Recently Smeltz, in US. application Ser. No. 532,973, filed Feb. 1,1966, described a novel tetramethyl lead distillation method whereintetramethyl lead in the pres ence of acetonitrile forms azeotropes thatdistill at temperatures below the normal boiling point of eithercomponent. Smeltz also disclosed that when a volatile thermal stabilizeris present in the starting solution, it forms a binary azeotrope withthe acetonitrile which codistills with the tetramethyl lead-acetonitrileazeotrope. However, in practice, it has been found that when stabilizingamounts of stabilizer are present in the feed, only a small fractionactually codistills with the tetramethyl lead, apparently due toalteration of volatilities when all of the components are presenttogether. Thus, a significant portion of the stabilizer does not appearin the distillate thereby reducing the amount of tetramethyl leadstabilization there and requiring recovery of substantial amounts ofstabilizer from the still bottoms.

Further, as Smeltz also disclosed, the distillate tends to separate intotwo liquid phases, the upper layer being poor in tetramethyl lead whilethe lower is rich in tetra methyl lead, often containing more than 85%tetramethyl lead. Because only a small amount of the stabilizercodistills With the tetramethyl lead and acetonitrile, an unsafesituation is created, there being insuflicient stabilizer in the lowerlayer to adequately stabilize the high tetramethyl lead content. It hasnow been found that such phase separation also tends to occur within thefractionating column itself. Thus, these azeotropic distillationspresent a serious explosion hazard, even though stabilizing proportionsof volatile stabilizer are initially present in the distillation feed.

Description 0] the invention It is an object of this invention toprovide an economical method for completely stabilizing tetramethyl leadduring fractional distillation of tetramethyl lead-acetonitrile azeo-3,362,889 Patented Jan. 9, 1968 "ice tropes. This and other objects willbecome apparent from the following description of this invention.

It has now been discovered that tetramethyl lead-acetonitrile azeotropescan be economically distilled under completely safe conditions by (a)Feeding into a fractional distillation unit containing a reboiler, afractionating column having at least about 3 theoretical plates and anoverhead distillate takeofi, a tetramethyl lead composition containingacetonitrile in azeotrope-forming proportions, at a point at least about1 theoretical plate above the reboiler,

(b) Simultaneously feeding at least about 0.25 part by weight, per partof tetramethyl lead in the feed composition, of a volatile thermalstabilizer for tetramethyl lead at a point not below the top of thecolumn and at least about \2 theoretical plates above the tetramethyllead composition feed point, whereby the stabilizer is distributedthroughout the distillation column in tetramethyl leadstabilizingproportions, and

(c) Recovering overhead a distillate which contains substantially allthe tetramethyl lead and stabilizer fed to the column and which, onseparating into two phases, produces layers each having a tetramethyllead to stabilizer weight ratio not greater than about 5:1.

In view of the teaching in the Smeltz application that the stabilizersform azeotropes with acetonit-rile which codistill with the tetramethyllead-acetonitrile azeotrope, it is quite surprising that the separatestabilizer feed technique of this invention significantly alters thedistribution of stabilizer throughout the system. In spite of the factthat azeotropes are being formed, as evidenced by distillation ofmixtures at temperatures below the boiling points of their components,it has been found in accordance with this invention that the compositionof the distillate can be varied substantially by the manner in which thestabilizer is introduced.

Although adding a large excess of stabilizer to the feed compositionmight also provide stabilizing proportions throughout the distillationunit and in the distillate, this is a costly expedient involving therecovery of large amounts of stabilizer from the bottoms stream. Infact, it has been found that to provide the 4:1 tetramethyl lead tostabilizer ratio desirable for complete stabilization throughout thedistillation, about one part of toluene per part of tetramethyl leadwould be needed in the feed composition. This means about of the addedtoluene would leave the column with the bottoms and would have to berecovered. In accordance with the present invention, the use of largeexcesses of stabilizer is avoided.

The figure schematically illustrates a typical embodiment for carryingout the process of this invention.

The tetramethyl lead compositions which are useful as feed material forthe process of this invention are those containing acetonitrile inazeotrope-forming proportions. When no water is present, distillation ofthese compositions under firactionating conditions results in theformation of a binary azeotrope containing about 61.2% by weighttetramethyl lead and about 38.8% acetonitrile and boiling at 73.3 C. atnormal atmospheric pressure. When water is present, a ternary azeotropecontaining about 62% by weight tetramethyl lead, about 31% acetonitrileof the nitrile up to about 100 parts. Suitable tetramethyllead-acetonitrile compositions can be prepared by adding the requiredamount of acetonit-rile to tetramethyl leadcontaining reaction productsobtained by the reaction of methyl chloride with sodium-lead alloy asdescribed by Iarvie et al. in US. Patent 3,048,160, by Tullio in US.Patent 3,072,694, and by Cook et al. in U.S Patent 3,049,558.

The process of this invention is particularly directed to the use of tetrarnethyl lead-acetonitrile compositions obtained by electrolyticallyproducing tetramethyl lead at a lead cathode as described by Silversmithand Sloan in US. Patent 3,197,392 and by Smeltz in copending US.application Ser. llo. 377,312, filed June 23, 1964. These processesproduce reaction mixtures containing unreacted methyl bromide,tet'ramethyl lead, acetonitrile, tetraalkyl ammonium bromide in whicheach alkyl group has 1-2 carbon atoms, and sometimes water. Normallybefore such mixtures are distilled in accordance with the presentinvention, the methyl bromide is first distilled off, usually in aseparate stripping still at a 3 to 4 C. head temperature. After removalof the methyl bromide these solutions generally contain about 1 to 12%by weight tetramethyl lead, about 40 to 85% aeetonitrile, about 530%tetraalkyl ammonium bromide and to 6% water.

Substantially all the tetramethyl lead present in the feed distills ofiin the form of the azeotrope. Since acetonitrile normally boils at 81.6C., any excess present stays behind under fractlonating conditions.Likewise any tetraalkyl ammonium bromide present in the originalcatholyte composition is practically non-volatile and accumulates in thestill bottoms.

Referring now to the figure, the tetramethyl lead-containing feedcomposition is pumped from tank 1 through heat exchanger 2 into themiddle region of a conventional fractional distillaiton unit containinga reboiler 3 surmounted by a fractionating column 4 which may containactual plates 5 or suitable packing. Stabilizer for the tetra- .ethyllead is pumped from storage tank 6 through heat exchanger 7 into the topof column 4.

The distillate take-elf unit illustrated in the figure is of the c-fisettype. Distillate is removed from the column through overhead distillatetakeoff line 8 and passed to condenser 9. The condensed vapors arepassed to decanter 19 where separation of the distillate into two layerstakes place. The upper layer, which is rich in acetonitrile, is passedthrough heat exchanger 11 and returned to the column. The lower layer,which is rich in tetram-ethyl lead, is recovered via line 12 as aproduct of the process. Bottoms stream, which is primarily acetonitrile,is withdrawn from the 'reboiler via line 13.

The fractionating column should have at least three theoretical platesand preferably 515, but may have as many as about 25 or more. The inletfor the tetramethyl lead feed composition should be at least 1 andpreferably 2-4 theoretical plates above the reboiler, so that thetetramethyl lead azeotropes can be efilciently stripped from theremaining feed.

The inlet for the stabilizer should be at a point not below the top ofthe column. The phrase top of the column refers to the uppermost platein the column or, if the column does not contain actual plates, the topof the packing. Thus the stabilizer may be added at the top of thecolumn or at some point above the top of the column such as the refluxzone. Of course the stabilizer could also be added with the distillaterecycle provided it is added at a point not below the top of the column.This stabilizer inlet point should also be at least 2 theo reticalplates, and preferably 37, but practically speaking not more than about15 theoretical plates above the tetramethyl lead feed composition inletpoint, whereby the stabilizer is substantially completely stripped fromthe system, with little or no stabilizer entering the bottoms.

The invention is illustrated below using toluene as the stabilizer.However, other known volatile stabilizers boiling in the to 160 C. rangecan also be used in accordance with this invention. Typical examples ofsuitable stabilizers include saturated and unsaturated aliphatic,alicyclic and aromatic hydrocarbons and halogenated hydrocarbons such ascyclohexane, n-heptane, Z-rnethylhexane, Z-methylhexane, heptene,methylhexene, n-octane, isooctane, octene, 3 ethylhexane, 2,5dimethylhexene, 2,2,5 trimethylhexane, 2,6-dimethylheptane, 4-ethylheptane, nonane, terpenes such as turpentine, benzene,ethylbenzene, styrene, the xylenes, ethylene dichloride, and mixturesthereof.

The stabilizer to tetramethyl lead ratio necessary for properstabilization varies with the stabilizer and is det'ermined by the usualthermal tests described in the Cook et al. patent cited above. Atetramethyl lead to stabilizer weight ratio not greater than about 5:1provides good thermal stability in most cases. Preferably the ratioshould be 4:1 or less.

Conveniently and preferably the process is conducted continuously, thestabilizer and tetrainethyl lead composition streams being fedcontinuously to the column, distillate being taken off overheadcontinuously and the bottoms stream being removed from the reboilercontinuously. Aiternatively, the bottoms may be allowed to accumulate inthe reboiler. The stabilizer and tetramethyl lead composition streamsshould be fed to the column simultaneously at rates that are consistentwith the capacity of the column so as to avoid flooding and provide atleast 0.25 part, and usually not more than about 0.4 part, stabilizerper part tetramethyl lead in the total feed. More than 9.4 part gives noparticular benefit, although larger amounts may be used if desiredwithout adverse efiect.

Heat for the distillation may be provided solely by the column and its.reboiler equipped with heating means or in part by the feed. Forexample, preheated tetramethyl lead composition can be fed to aninsulated column, and the temperature and/ or pressure within the columncan be adjusted to cause the appearance of the azeotropic composition inthe overhead. The stabilizer, which may be preheated if desired, can befed to the column as liquid or injected as vapor.

Usually the distillation is conducted at about atmospheric pressure andhead temperatures of about 69 to C. Most commonly temperatures of about72 to 77 C. are used. Reduced pressures and correspondingly lowertemperatures may also be used, for example mm. Hg (0.2 atmosphere) and35 C.

An offset type distillation head is conveniently used. Distillatecontaining tetramethyl lead, a stabilizing proportion of stabilizer,acetonitrile and water, when present, is taken oif continuousiy, usuallyat a reflux ratio of about 3:1 to 15:1 and preferably about 4:1 to 8:1.Distillates containing at least about 39% by weight tetramethyl leadseparate into two distinct liquid phases at or just below the boilingpoint of the azectrope. The upper layer is poor in tetramethyl lead,while the lower layer is rich in tetramethyl lead. The thermalstabilizer and water, when pres ent distribute themselves between thetwo layers.

The tetramethyl lead can be recovered from the total distillate bywashing the distillate with water, which extracts the acetonitrile,leaving the tetramethyl lead and the thermal stabilizer. Preferably thedistillate is first allowed to separate into two layers and thetetramethyl leadpoor upper layer is continuously returned to the column0 after reheating back to column temperature. At the same time, thelower tetramethyl lead-rich layer is continuously removed and washedwith water to remove residual nitrile.

The bottoms stream is preferably removed continuous- 1y from thereboiler at a rate which is substantially the difference between thetotal feed rate and the distillate take otl rate. This stream issubstantially tetrarnethyl leadfree and stabilizer-free.

The overall result achieved in accordance with this invention is thatsubstantially all the tetramcthyl lead and all the stabilizer fed to thecolumn is removed overhead.

Thus a tetramethyl lead-stabilizer composition is obtained directly inthe desired proportions while substantially no tetramethyl lead orstabiizer is lost to the bottoms stream. Moreover, samples taken fromthe column during distillation show that within the coluumn itself nounstable tetramethyl lead compositions exist. In the lower regions ofthe column where the stabilizer concentration is lowest, the overallcompositions are so dilute in tetramethyl lead, due to the relativelylarge proportion of acetonitrile, that they present no explosion hazard.

Further up the column, in the middle and top regions, the tetramethyllead content increases progressively due to the column enriching action.However, the stabilizer content also increases and, due to thestabilizer feeding technique of this invention, shows a progressivelygreater concentration gradient than the tetramethyl lead so that theamount of stabilizer corresponds to a stabilizing proportion at alltimes. Accordingly, tetramethyl lead to stabilizer ratios of not greaterthan :1, and preferably 4:1 or less, are readily maintained throughoutthe column and in the distillate.

The following examples, illustrating the novel process disclosed herein,are given without any intention that the invention be limited thereto.All parts and percentages are by weight.

EXAMPLE 1 The basic distillation unit used in this example contained afractionating column, reboiler and variable reflux ratio distillationhead. The column was a vacuum jacketed, Oldershaw cylindrical glasscolumn having a one-inch inside diameter and 15 real plates spaced oneinch apart, essentially as described by Collins et al. in Ind. and Eng.Chem. Qual. Ed., 18, No. 11, 673 (1946). The column was further fittedwith sampling taps at every other plate, a feed stock inlet at fiveplates above the reboiler, and a stabilizer feed inlet above theuppermost plate and below the condenser. An automatic buret leading tothe column through a microbellows pump and heat exchanger served tostore, meter, and preheat the stabilizer to feed temperatures.

The reboiler was a 0.5 liter flask fitted with heating jacket, siphondraw-off tap for removing bottoms, and thermocouple well. Thedistillation head was equipped 6 With a timer-operated plunger forcontrolling distillate takeotf.

The plate efliciency of the column was found to be 25 to 30% using thevapor-liquid equilibrium data for acetonitrile-water mixtures of Othmeret al. in Ind. and Eng. Chem, 39, 1175-1177 (1947). This efliciencyrating corresponds to about 2.5-3 theoretical plates separating thetetramethyl lead and stabilizer feed points.

A tetramethyl lead-acetonitrile-water composition, as described in thetable below, was preheated to C., and fed to the column through the feedstock inlet at a 20 gms./ min. rate. Simultaneously toluene, preheatedto 70 C., was fed to the top of the column at a 0.32 gm./min. rate,corresponding to a 4:1 tetramethyl lead to toluene weight ratio. Thereboiler was heated to provide a bottoms temperature of about 79 C. Withthe column operating at a 74 C. head temperature and a 10:1 refluxratio, 2 grams of distillate per minute were taken off overhead whilesimultaneously 10 grams of bottoms per minute were removed from thereboiler.

The total composition of the distillate is shown in the table. Oncooling, this distillate separated into an upper tetramethyl lead-poorlayer and a lower tetramethyl lead-rich layer. The total distillatecontained tetramethyl lead and toluene in about a 4:1 weight ratio.

A comparative run, not within the scope of this invention, was made byrepeating the above experiment except that the toluene was combined withthe tetramethyl lead feed stock and introduced into the column at thetetramethyl lead composition feed point. The tetramethyl lead content ofthe feed stock and the tetramethyl lead to toluene feed ratio wereslightly lower than in the above example as indicated in the table,which should have had a favorable influence on the result. Thetetramethyl lead to toluene weight ratio of the resulting totaldistillate was an unsafe 9:1.

EXAMPLE 2 Example 1 was repeated except that the height of the columnwas increased to 20 real plates and other variables were changed asindicated in the table. The toluene feed point was 3.7 5-4.5 theoreticalplates above the tetramethyl lead feed point.

TABLE Invention Examples Conditions Comparative Run Feed Composition,Percent' TML 3.4 6 6 6 CHsCN--. 86. 6 9O 90 2O 9.0 4 4 4 Toluene 1.0Real Plates 15 15 20 20 TML Composition Feed Rate, gmsJmin.-. 22 20 2020 Toluene Added at Head N 0 Yes Yes Yes Toluene Feed Rate at Head,grns.lmin 0 0. 32 0. 32 0. 32 TML/Toluene Feed Ratio..- 3. 4/1 4/1 4/14/1 Reflux Ratio 10/1 10/1 4/1 10/1 Distillate Recovery Rate,gms./miu 1. 6 2 3 2. 2 Bottoms Recovery Rate, gms./miu- 20.4 18 17 17.8Upper Layer Recycled N0 N o No Y Head Temperature, C 72 74 73 76 ToluenePreheat Temperature, C 70 70 66 Reboiler Temperature, C 78. 6 79 84 79Layer Layer Layer Results Lower Layer Lower Upper Total Lower UpperTotal Lower Upper Total 36.3 40 69. 3 50. 3 29 12. 2 9. 4 15 1. 2 4. 014 17. 3 9/1 3/1 4/1 \IL 0. 004 0. 07 0. 1 CH CN 89. 8 96.1 95. 8 H2O 9.5 3. 7 3. 9 Toluene 0. 6 0. O8 0. 17 Toluene Recovery Ra 0.5/1 31/1 12.5/1

7 EXAMPLE 3 Example 1 was repeated except that a modified fractionaldistillation unit was used and the operating variables were changed asindicated in the table. The height of the column was increased to 20real plates and the condenser was an off-set type mounted in anoil-center extension from the top of the distillation column. Thedistillate was allowed to stratify, the lower tetramethyl lead-richlayer was recovered and analyzed, and the upper tetramethyl lead-poorlayer was recycled to the top of the column via a 70 C. heat exchanger,by means of a microbellows pump.

The table above, in which TML represents tetramethyl lead, summarizesthe above experiments and results.

From the above table, it can be seen that the toluene recovery ratio(toluene in distillate/toluene in bottoms) was dramatically improvedwhen toluene was fed near the top of the column in accordance with thisinvention. In contrast, conventional toluene feed in the Comparative Runresulted in about half of the toluene being lost to the bottoms.

The above data also shows that introducing the toluene with thetetramethyl lead feed stock (Comparative Run) does not providesulficient toluene in the distillate to ensure complete tetramethyl leadstabilization. The 8:1 tetramethyl lead to toluene ratio in the lowerlayer of the Comparative Run presents a hazardous situation. Moreover,this explosion hazard exists in the column itself since phase separationalso occurs there, particularly at the uppermost plates. In contrast,tetrarnethyl lead to toluene ratios are readily maintained within safelimits both in the column and the distillate in accordance with thepresent invention. In actual practice, Example 1 would be run with atetramethyl lead to toluene feed ratio of about 3.4:1, as in theComparative Run, which would ensure completely stable conditions in thetetramethyllead-rich lower layer of the distillate.

EXAMPLE 4 The tetrarnethyl lead feed stock employed in this example wasobtained from a tetramethyl lead-rich catholyte produced according tothe electrolytic process described by Smelt in copending application,Ser. No. 377,312, filed June 23, 1964. The original catholyte leavingthe Smeltz cell contained 5% tetramethyl lead, 57% acetonitrile, 3%water, 20% tetraethyl ammonium bromide and methyl bromide. The methylbromide content was distilled oil in a conventional distillation columnand the residue was used as feed stock for the distillation inaccordance with this invention.

The feed stock containing 6% tetramethyl lead, 69.2% acetonitrile, 5.3%water and 19.5% tetraethyl ammonium bromide, was preheated to 65 C. andfed at a 4 gaL/hr. (ca. 35 lbs/hr.) rate to the mid point of a lagged 4in. X 10 ft. 316 stainless steel column packed with A in. Intaloxceramic saddles and having 10 theoretical plates. The column was fittedwith a gal. 316 stainless steel reboiler and a 316 stainless steel shelland tube type condenser. Simultaneously toluene at C. was fed at the topof the column at a 0.5 lb./ hr. rate. The total feed was al lowed todistill in the column at a reflux ratio of about 10:1. The tetramethyllead-poor upper layer of the distillate was recycled to the column whilethe tetramethyl t3 lead-rich lower layer was recovered at the rate of2.5 to 3 lbs/hr. A bottoms stream was removed from the reboiler at therate of 32 to 33 lbs/hour.

The recovered distillate contained 76% tetramethyl lead, 18% toluene,acetonitrile and 0.5% water for a tetramethyl lead to toluene ratio ofabout 4:1. Washing with water removed the nitrile and residual water,leaving a stabilized tetramethyl lead-toluene composition suitable forantiknock blending. The bottoms contained 69% acetonitrile, 6% water and26% tetraethyl ammonium bromide with no detectable tetramethyl lead ortoluene.

Although the invention has been described and exemplified by way ofspecific embodiments, it is not intended that it be limited thereto. Forinstance, other stabilizers than toluene could be used in the aboveexamples with essentially the same result. It is to be understood thatthe invention includes all modifications and variations coming withinthe scope of the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of distilling tetramethyl lead-acetonitrile azeotropes whichcomprises (a) feeding into a fractional distillation unit containing areboiler, a fractionating column having at least 3 theoretical platesand an overhead distillate takeoii, a tetramethyl lead compositioncontaining acetonitrile in azeotrope forming proportions at a point atleast 1 theoretical plate above the reboiler.

(b) simultaneously feeding at least 0.25 part by weight,

per part of tetramethyl lead in the feed compositlcn, of a volatilethermal stabilizer for tetramethyl lead at a point not below the top ofthe column and a least 2 theoretical plates above the tetramethyl leadcomposition feed point, whereby the stabilizer is distributed throughoutthe distillation column in tetra- .methyl lead-stabilizing proportions,and

(c) recovering overhead a distillate which contains substantially allthe tetramethyl lead and stabilizer fed to the column and which, onseparating into two phases, produces layers each having a tetramethyllead to stabilizer weight ratio not greater than 5:1.

2. The method of claim 1 in which the tetramethyl lead composition isfed into the fractional distillation unit 2 to 4 theoretical platesabove the reboiler and the thermal stabilizer is fed 3 to 7 theoreticalplates above the tetramethyl lead composition feed point.

3. The method of claim 2 in which the thermal stabilizer is toluene.

4. The method of claim 3 in which 0.25 to 0.4 part of toluene is addedper part of tetramethyl lead in the feed.

References Cited UNlTED STATES PATENTS 2,247,255 6/ 1941 Senkus 20362,543,575 2/1951 Harvey et a1. 2036 2,990,340 6/1961 Barnes 20362,996,351 8/1961 S-tobe 203-6 3,048,610 8/1962 Iarvie et a1. 2604373,197,392 7/1965 Silversmith 204-59 3,221,039 11/1965 Cook et a1.260-437 WILBUR L. BASCOMB, In, Primary Examiner.

